There is a present need for a pipe thread sealant paste that is thermally stable and of high enough purity level to meet the requirements of the nuclear power industry. Most commercially available pipe thread sealant pastes are made from materials that decompose at temperatures which are too low to be useful in "fire-safe" pipe joints required in nuclear power plants. Moreover, these known sealant pastes often contain impurities which may be harmful to metal pipes. Chlorine, for example, is known to cause embrittlement of stainless steel pipes even when present in very small amounts. Other impurities which may be detrimental and harmful include sulfur, fluorides, nitrates, nitrites, zinc, for example, and other embrittling metals.
Graphite is a thermally stable material which has heretofore been used as a filler component in various kinds of lubricants, coatings, cements, packing materials and the like. For example, in U.S. Pat. No. 1,594,810 to Benner et al; there is disclosed a non-corrodible, conductive, thermoplastic coating composition for dry cell electrodes comprising resin, linseed oil, scale wax and powdered graphite. Also, in U.S. Pat. No. 2,270,199 to Thrune, a graphite paste composition useful as a cement for joining solid graphite bodies is disclosed wherein electric furnace graphite powders are blended with a liquid coal tar and a hardening agent which is reactive with the coal tar at elevated temperatures. Similarly, in U.S. Pat. No. 3,928,214 to Naka et al, a lubricating grease for electrical components is disclosed comprising a mineral oil having dispersed therein a high melting wax and a non-volatile solid lubricant such as graphite. None of these prior art references, however, disclose applications where the graphite is employed as a sealant at very high temperatures.
U.S. Pat. No. 4,162,078 to Cox, discloses an injectible packing formulation comprising flexible graphite particles and a liquid lubricant. The flexible graphite particles are prepared by shredding flexible graphite sheet made according to known methods by expanding graphite flakes by a factor of at least 80 times in the "c" crystallographic axis direction, and then compressing the expanded flakes to form a cohesive structure. The shredded flexible graphite particles employed in the packing formulation are fairly large in size varying in length up to about 0.250 inch. The smallest particles used in the formulation are of a size small enough to pass through a 30 U.S. Standard sieve (0.0234 inch). The liquid lubricant used in the formulation may be a synthetic oil such as a silicone fluid and is employed in amounts ranging from about 5 percent by weight to about 35 percent by weight based on the total weight of the flexible graphite and lubricant. Because the flexible graphite particles are thermally stable at elevated temperatures in excess of about 500.degree. F., the packing formulation can be used in many high temperature applications such as steam expansion joints, for example.
It has been proposed to employ as a pipe thread sealant a flexible graphite tape produced under the trade name "Grafoil" by Union Carbide Corporation of Danbury, Conn. This tape is similar to the flexible graphite sheet described in the Cox patent, supra. Although the flexible graphite tape is thermally stable and should be satisfactory for use as a high temperature pipe thread sealant, the tape is relatively thick (approximately 0.006 inch) and cannot be used with stainless steel pipe in small diameters because there is not enough space within the pipe threads to accommodate the relatively thick tape. Also, pipe fitters generally prefer to use a paste because it is a common method of sealing pipe threads.
Pipe thread sealant paste prepared from natural flake graphite particles and a liquid carrier such as linseed oil has some drawbacks. Specifically, the linseed oil may polymerize and thereby harden in use to a point where it is extremely difficult to separate the pipe if required. A more important practical drawback is that it must be stirred before use because the graphite particles tend to settle to the bottom of the suspension. Where high purity sealant applications are required, then a non-contaminated stirrer would have to be used. The use of a contaminated article as a stirrer, could affect the joint seal in some applications.
It is an object of this invention to provide a high purity pipe thread sealant paste.
It is another object of this invention to provide a high purity, high temperature sealant paste that can be packaged in a tube and which can be used from the tube for easy application to the joint of a pipe.
It is another object of this invention to provide a high purity, high temperature sealant paste comprising a substantially homogeneous mixture of natural flake graphite particles with a mineral oil and petrolatum.